Method and apparatus for ion mobility spectrometry

ABSTRACT

Molecular ions are generated by ionization, said molecular ions are accumulated in an ion reservoir that is external to the drift chamber. Than said molecular ions are dissociated into fragment ions (i.e. fragmented ions) with electromagnetic radiation or electron beams or ion beams, and said fragment ions are ion-mobility spectrometrically analyzed. In an embodiment the apparatus comprises additionally a virtual impactor and a pyrolyzer. The process of fragmentation over time are detected and analyzed, and this information is used for the differentiation of hazardous biological samples from non-hazardous biological samples.

BACKGROUND OF THE INVENTION

The present invention relates to an improved method and apparatus forion mobility spectrometry. In particular the invention provides a methodand apparatus that yield a higher information content of the obtainedion-mobility spectra and a better probability of correct identificationof hazardous substances and a better distinction between hazardous andnon-hazardous chemical and biological agents. The method and apparatusof the invention can be used for the analysis of ions of macromoleculesfor environmental screening, e.g. the detection of proteins and lipidsthat occur in hazardous biological agents. In particular the improvedmethod and apparatus for ion mobility spectrometry are useful for thedetection of biological weapons made from viruses or bacterial sporesand inorganic and organic surfactants and other chemicals, e.g.micrometer-sized dust-forming silicate particles.

Ion mobility spectrometry is a powerful analytical tool for thedetection of chemical and biological hazards. Typically, in an ionmobility spectrometer (IMS) the sample is ionized, passed through anelectric field and the time-of-flight of the different sample ions atatmospheric pressure is detected by an electrode detector. Thedisadvantage of these prior art IMS is that the false alarm rate for thedetection of some chemical and biological hazards is too high for manyimportant civil applications. Some mass spectrometers (MS) have betterfalse alarm rates, but MS are very expensive since they requirecomplicated vacuum technology (see e.g. U.S. Pat. No. 6,342,393). Thepurpose of this invention lies in an improved method and apparatus forion mobility spectrometry to obtain a significantly improved accuracy ofdetection.

BRIEF SUMMARY OF THE INVENTION

In a first embodiment, an IMS is set out wherein molecular ions of thesample are dissociated into fragment ions, and in which the spectra ofthe fragment ions and the process of fragmentation over time areanalyzed. For example, electromagnetic or electron beams may createfragmentation which increases the number of different ions that aredetected by the detector of the ion-mobility spectrometer. For detectionof biological hazards, the sample may be collected by a virtualimpactor, partially chemically decomposed in a pyrolyzer and separatedinto fractions in gas chromatograph before being analyzed in the IMS. Ina further embodiment of the methods and apparatuses of the invention,the interaction of the sample ions with each other over time ismonitored and used to achieve a higher information content. Beyond this,in one embodiment of the methods and apparatuses of the invention, achemical that interacts with the sample is added to the inert gas of theion mobility spectrometer and the changes of the ion mobility spectraare monitored and used for obtaining a higher information content. Thischemical can e.g. be a pH-modifier. Beyond this, in another embodimentof the methods and apparatuses of the invention, larger particles aredetected with an ion-mobility spectrometer by using the reversion of theflow of the inert gas relative to the common direction and therebydragging large particles towards the collector electrode, and using thisdetection to obtain a higher information content about the sample, e.g.about the presence of weapons-typical additions to spores and viruses.In the embodiments which comprise multiple gatings, before injecting anew sample into the ionization chamber, a higher yield of collected ionsmay be achieved which may lead to a further improvement of signal/noiseratios. Said ion mobility spectrometers may be operated in the positiveor negative ion mode or in both ion modes. The ionization of a targetcompound of the sample can be done directly by an ionization source thatemits energy that interacts with and ionizes the target compound.Alternatively or additionally, a target compound of the sample can beindirectly ionized by an ionization source which emits energy thatinteracts with and ionizes an intermediate compound which, in turn,interacts with and ionizes the target compound. It should be understoodthat his invention has been disclosed so that one skilled in the art mayappreciate its features and advantages, and that a detailed descriptionof specific components and the spacing and size of the components is notnecessary to obtain that understanding. Many of the individualcomponents of the ion mobility spectrometers are conventional in theindustry, and accordingly are only schematically depicted. Thedisclosure and description of the invention and the examples are thusexplanatory, and various details in the construction of the equipmentare not included. Alternative embodiments and operating techniques willbecome apparent to those skilled in the art in view of this disclosure,and such modifications should be considered within the scope of theinvention, which is defined by the claims. The invention described canof-course also be used in combination with the known prior art variantsof ion-mobility spectrometry.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be described in conjunction with certain drawingswhich are for the purpose of illustrating the preferred and alternateembodiments of the invention only, and not for the purpose of limitingthe same, and wherein:

FIG. 1 is a schematic structural view showing of an apparatus for ionmobility spectroscopy with a infrared laser for fragmentation;

FIG. 2 is a schematic structural view showing of an apparatus for ionmobility spectroscopy with a UV lamp for fragmentation;

FIG. 3 is a schematic structural view showing of an apparatus for ionmobility spectroscopy with a infrared laser for fragmentation and withseveral gating pulses;

FIG. 4 is a schematic structural view showing of an apparatus for ionmobility spectroscopy with a infrared laser for fragmentation andinteraction with chemical addition;

FIG. 5 is a block diagram of an apparatus for ion mobility spectroscopy;

FIG. 6 is a schematic structural view showing of an apparatus for ionmobility spectroscopy with 30 guard rings and 2 pumps;

FIG. 7 is a schematic structural view showing of an apparatus for ionmobility spectroscopy with an impactor and a pyrolyzer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic structural view showing of an apparatus for ionmobility spectroscopy with a infrared laser for fragmentation. Thesample 1 is injected into the ionization chamber 2 and ionized by thesource of ionization 3 which may be e.g. a radioactive source such as³H, ⁵³Ni, or ²⁴¹Am, UV or VUV light, or an electrical discharge(non-radioactive electron source). For example, when using a ⁵³Ni foilas source of ionization and air as drift gas, the primary ions aremainly short-living N₂ ⁺, NO⁺ and O₂ ⁺. These N₂ ⁺, NO⁺ and O₂ ⁻ rapidlyreact with traces of water in the drift gas to form clusters of thetypes N₂ ⁺(H₂O)_(x), NO⁺(H₂O)_(y), and O₂ ⁺(H₂O)_(z), which then clusterwith the molecules and clusters of the sample. The ionization chamber 2serves as ion reservoir. After injection of the sample into theionization chamber 2, some of the sample molecules and sample ions startto dissociate into fragment molecules and fragment ions due tointeraction with the light from an infrared LASER 4. With the help of agating pulse which is applied to the gate 5, the fragment ions 6 fromthe ionization chamber 2 are transferred into the drift chamber 7 wherethe fragment ions 6 are accelerated by an electric field 8. The time offlight of the fragment ions 6 in the gaseous phase is measured with thehelp of a collector 9. Since different fragment ions 6 have differentmobilities in the gas of the drift tube, they result in distinct peaksin the IMS spectrum 10. Several measurements, without interrupting thedissociation reaction caused by the LASER 4, are done before injecting anew sample into the ionization chamber 2. The indicated time points, 0,100 ms, and 200 ms, respectively, refer to the time after application ofa gating pulse. In order to reduce the noise, the ion mobilityspectrometer is enclosed in a grounded copper foil. The collector 9 isconnected with a 10¹⁰-V/A pre-amplifier via a cable of only a few mmlength. The feedback resistor of the pre-amplifier was selected for alow noise level. The voltage supply for the guard rings of the drifttube is stabilized to better than 0.1% rms, a) after a short period oftime for fragmentation, e.g. 1 second, b) after a long time offragmentation, e.g. 5 seconds; significant fragmentation has occurredand accordingly characteristic changes of the heights of some peaks inthe ion-mobility spectra are observed. The information of the spectra atthe beginning, i.e. when the sample is still non-fragmented, and thecharacteristic changes of spectra over time due to fragmentation areused to characterize the sample. In particular, the sample is evaluatedfor a hazardous biological content. For example, the measured spectraare correlated with a data base which contains spectra of non-fragmentedsamples and their changes due to fragmentation wherein the correlationprocedure includes the use of small distortions of the drift time of thespectra.

The operation of the apparatus in FIG. 2 is similar to FIG. 1, but herethe fragmentation is done with light from an UV or vacuum-UV (VUV) lamp11. Alternatively, the fragmentation may be done with electron beams(electron-ionization or electron bombardment) or ion beams (ionbombardment) or other methods. For example, electron beams may begenerated in vacuum and released into the gaseous medium of theionization chamber 2 through a thin membrane. The indicated time points,0, t₁, and t₂, respectively, refer to the time after application of agating pulse. Depending on the methods of ionization and fragmentation,the amount of humidity in the drift chamber 7 may greatly affect thesensitivity of the spectrometer. That is why in some variants of thisdesign, the humidity may be controlled by pumping the drift gas througha molecular sieve. After a short period of time for fragmentation (a),the spectra show little change. After a long time of fragmentation (b),significant fragmentation has occurred and accordingly characteristicchanges of the heights of some peaks in the ion-mobility spectra areobserved. The operation of the spectrometer may comprise the followingsteps: (i) The sample is continuously collected from different locationsvia a pump and several tubes with 2 mm diameter and a few m length. (ii)The sample 1 is passed through a virtual impactor which selects a sizerange of 0.5-8 μm and discards particle sizes which are smaller than 0.5μm and larger than 8 μm. (iii) The collected sample is stored in acontainer having a 20 mL volume. (iv) After 2 minutes of collection andstorage, the complete sample is transferred from the container into apyrolyzer which causes partial decomposition of the sample. (v) Theproduct of the pyrolyzation reaction at 350° C. within the time range of5 s-8 s after transfer to the pyrolyzer is transferred to the ionizationchamber of the ion-mobility spectrometer. (vi) In the ionization chamber2, the sample is ionized and fragmented. (vii) The first gating pulse isapplied a few milliseconds after transfer of the sample to the gate 5.(viii) The first ion-mobility spectrum is recorded and stored on acomputer. This spectrum corresponds to the essentially non-fragmentedsample. (ix) Several more ion-mobility measurements are performed on thesample over a period of 30 seconds. The spectra obtained correspond todifferent degrees of fragmentation of the sample and are also stored onthe computer. (x) By this way the spectra of the sample with differentdegrees of fragmentation, from essentially non-fragmented to essentiallycompletely fragmented, are obtained. (xi) The information from thespectrum of the almost non-fragmented sample and the information fromthe transitions of several peaks in the course of fragmentation are usedfor the analysis of the sample. In particular neuronal networks are usedfor the distinction between hazardous and non-hazardous samples. Becausethe information content of the spectra is much higher than in the priorart ion mobility spectrometry of biological agents, the false alarm rateis significantly reduced.

FIG. 3 shows a schematic structural view showing of an apparatus for ionmobility spectroscopy with a infrared laser for fragmentation and withseveral gating pulses. The sample is injected into the ionizationchamber 2 and ionized by the source of ionization 3 which may be e.g. aradioactive source such as e.g. ³H, ⁵³Ni, or ²⁴¹Am, UV or VUV light, oran electrical discharge (non-radioactive electron source). Afterionization, some of the sample ions start to interact with each other12. After the fragmentation several gating pulses are applied to thegate 5 and several measurements of ion-mobility spectra 10 are madebefore a new sample is injected into the ionization chamber 2. Thus,successive ion-mobility spectra follow the interaction of ions in theionization chamber 2. This change of the spectra over time is used for abetter characterization of the sample, a) after a short period of timeof interaction, b) after a long time of interaction in the ionizationchamber 2, characteristic changes of the heights of some peaks in theion-mobility spectra are observed and used for the identification of thesample.

FIG. 4 shows a schematic structural view showing of an apparatus for ionmobility spectroscopy with a infrared laser for fragmentation andinteraction with chemical additions. The biological sample is injectedinto the ionization chamber 2 and ionized by the source of ionization 3which may be a radioactive source such as e.g. ³H, ⁵³Ni, or ²⁴¹Am, UV orVUV light, or an electrical discharge (non-radioactive electron source).A chemical addition was added to the inert gas of the IMS or already tothe sample in the pyrolysis tube. The chemical addition may be e.g. HClor NH₃. This chemical addition can interact 13 with the sample moleculesand sample ions and thereby causing specific changes of the ion-mobilityspectra 3 of the fragmentation ions. In particular, in the presence ofsome water vapor, NH₃ can bind to fatty acids of virus envelopes. Acidicadditions, e.g. HCl, and basic additions in the presence of some watervapor, can change the pH of proteins and polypeptides, and consequentlytheir charge state and thus their ion-mobility spectra. The changes ofthe ion-mobility spectra caused by the presence of the chemicaladditions help to identify and characterize the sample.

FIG. 5 shows a block diagram of an apparatus for ion mobilityspectroscopy. A virtual impactor 20 is e.g. continuously operated andserves for selecting and concentrating a certain size range ofparticles, e.g. 0.2 μm (e.g. single influenza virus or other pathogen)to 10 μm (e.g. several spores of anthrax bound to a dust particle orother pathogen). Particle size and size distribution offer too littleinformation to unambiguously identify biological hazards in the presenceof significant amounts of interferrents of non-hazardous substances.That is why the biological agents are collected and, from time to time,injected into a pyrolyzer 21 where they are partially decomposed intochemical components. The output from the pyrolyzer 21 is thentransferred to the ion mobility spectrometer (IMS) 22 where it isionized and further decomposed. In this way a very detailed ion-mobilityspectrum with a large number of peaks is obtained which represents afinger print of the biological agent. Sample injection into thepyrolyzer and sample transfer from the pyrolyzer to the IMS areorganized in such a way that a chemical pre-selection is performed, i.e.that only some of the products of the pyrolysis are analyzed in the IMS,e.g. lipids, polysaccarides, and weapons-typical additions to bacterialspores. A computer 23 analyzes the IMS spectra and as well their changesdue to fragmentation of ions. In this way a large amount of informationabout the biological agents is obtained which allows the distinctionbetween hazardous and non-hazardous agents.

FIG. 6 shows an ion mobility spectrometer with 30 guard rings 32 and 2pumps. A first pump 30 conveys the sample into the ionization chamber 2.The operation of an inert gas pump 34 at the other end of the driftchamber 7 can be reversed which leads to the possibility of detection ofvery large particles, e.g. weapons-typical micrometer-sized additions tobacterial spores. The insulating layers between the guard rings 32 aremade from an inert polymer. Guard rings 32 and insulating layers areheld together with 3 screws which each attached to a spring in order toexert a constant pressure on the guard rings 32 and insulating layers.

FIG. 7 shows a schematic structural view showing of an apparatus for ionmobility spectroscopy with an impactor and a pyrolyzer. For example, atwo-stage virtual impactor 20 is operated with a flow rate of a few 100L/min and collects and concentrates particles with sizes from about 0.2to 10 micrometers. The concentrated aerosol is transferred to thepyrolyzer 21 which is operated at about 350° C. After application of afew seconds of pyrolysis, the partially decomposed sample enters theionization chamber 2 of the ion-mobility spectrometer 22. The source ofionization 3, e.g. a ⁵³Ni foil, serves for the ionization of thepartially decomposed sample. The sample ions formed by this process arethen fragmented with electron beams which are generated by the electronbeam generator 14. Sample ions and fragment ions interact with eachother and form various clusters. The fragmentation and clusteringprocesses cause specific changes of the ion-mobility spectra 10 overtime. Several ion-mobility spectra in the positive and negative ionmodes are recorded before a new sample is injected into the ionizationchamber 2. The specific changes of the spectra over time are used forthe automatized differentiation of hazardous from non-hazardous sampleswith the help of software and computer 23.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for ion-mobility spectrometry of asample, comprising the steps of: (a) generating molecular ions byionization and accumulating said molecular ions in an ion reservoir thatis external to a drift chamber of an ion-mobility spectrometer; and (b)exposing said molecular ions in said ion reservoir to a source of energyfor a time sufficient for dissociation of said ions into fragment ionsprior to ion-mobility analysis in the drift chamber of the ion-mobilityspectrometer, wherein said exposing results in production of multiplecharge states of fragment ions, and a degree of said dissociation ofsaid ions into fragment ions is more than 50% for some of said ions andchanges of ion-mobility-spectra due to fragmentation by said more than50% are used for characterization of the sample.
 2. The method of claim1, wherein said source of energy comprises one of UV radiation, VUVradiation, and photo ionization.
 3. The method of claim 1, wherein saidsource of energy comprises one of electron beams, electron bombardment,electro spray, electron-ionization, corona discharge, and glowdischarge.
 4. The method of claim 1, wherein said source of energycomprises one of ion beams and ion bombardment.
 5. The method of claim1, wherein said source of energy comprises radioactive emission.
 6. Themethod of claim 1, wherein said source of energy comprises plasma. 7.The method of claim 1, wherein said source of energy comprises infraredradiation.
 8. The method of claim 1, wherein said exposing of said ionsin said ion reservoir to a source of energy is performed duringaccumulation of said ions in said ion reservoir.
 9. The method of claim1, wherein said exposing of said ions in said ion reservoir to a sourceof energy is performed after accumulation of said ions in said ionreservoir.
 10. The method of claim 1, wherein said source of energy iselectron beams which are generated in vacuum and released to non-vacuumparts of said ion-mobility spectrometer through a membrane.
 11. Themethod of claim 1, wherein said source of energy comprises UV radiationhaving a wavelength between about 120 nm and about 300 nm.
 12. Themethod of claim 1, wherein said source of energy comprises coherentinfrared radiation.
 13. The method of claim 1, wherein said ion-mobilityspectrometer is connected with a pyrolyzer or gas chromatograph.
 14. Themethod of claim 13, wherein only select fractions of the output of saidpyrolyzer or gas chromatograph are transferred to said ion-mobilityspectrometer.
 15. The method of claim 13, wherein said pyrolyzer or gaschromatograph is connected with a virtual impactor.
 16. The method ofclaim 1, wherein the drift chamber of said ion-mobility spectrometer isoperated at a pressure between about 500 and about 5000 torr.
 17. Themethod of claim 1, wherein the drift chamber of said ion-mobilityspectrometer is operated at a pressure between about 10 and about 500torr.
 18. The method of claim 1, wherein the generation of ions of thesample in said ion-mobility spectrometer is achieved using a radioactivesource comprises one of ³H, ⁵³Ni, ²⁴¹Am, UV light, VUV light, anelectrical discharge, a corona discharge, and electrospray.
 19. Themethod of claim 1, wherein said generating said molecular ions byionization involves an ionization of inert-gas molecules of saidion-mobility spectrometer and clustering of inert-gas ions with one ofsample molecules, sample-molecule clusters, and sample-moleculefragments.
 20. The method of claim 1, wherein said ion-mobilityspectrometer has a drift chamber with a length between about 40 cm andabout 60 cm.
 21. The method of claim 1, wherein the substance which isanalyzed in said ion-mobility spectrometer is pyrolyzed bioweapons-gradematerial.
 22. A method for ion-mobility spectrometry of a sample,comprising the steps of: (a) generating molecular ions by ionization andaccumulating said molecular ions in an ion reservoir that is external toa drift chamber of an ion-mobility spectrometer; (b) exposing saidmolecular ions in said ion reservoir to a source of energy for a timesufficient for dissociation of said ions into fragment ions prior toion-mobility analysis in the drift chamber of the ion-mobilityspectrometer, wherein said exposing results in production of multiplecharge states of fragment ions, and wherein further micrometer-sized andsub micrometer-sized particles are detected according to the followingsteps: (c) generating particle ions by ionization; (d) extracting gasfrom the drift chamber whereby movement of said particle ions towards acollector in the drift chamber of said ion-mobility spectrometer isincreased; and (e) measuring and analyzing the collector currentgenerated by particle ions.
 23. The method of claim 22, wherein saidparticles have sizes between about 100 nm and about 10 μm.
 24. Themethod of claim 22, wherein said particles have sizes between about 2 μmand about 10 μm.
 25. The method of claim 22, wherein said particles arebioweapons-grade micrometer-sized particles with attached spores orviruses.
 26. The method of claim 22, wherein said particles arebioweapons-grade silicate particles with attached spores or viruses. 27.The method of claim 22, wherein said particles comprise of inorganiccompounds that are partially coated with organic compounds.
 28. Themethod of claim 22, wherein said particles comprise at least onepathogen.
 29. The method of claim 22, wherein the sample is passedthrough a virtual impactor prior to analysis in said ion-mobilityspectrometer.
 30. The method of claim 22, wherein said generating saidparticle ions by ionization further comprises an ionization of inert-gasmolecules of said ion-mobility spectrometer and clustering of inert-gasions with said particle.
 31. The method of claim 1, wherein an electricfield of said drift chamber has a strength between about 50 V/cm andabout 5000 V/cm and is applied using more than 5 electrodes or guardrings.
 32. The method of claim 1, wherein said ion-mobility spectrometeris connected with a pyrolyzer and gas chromatograph.
 33. The method ofclaim 13, wherein only select fractions of the output of said pyrolyzerand gas chromatograph are transferred to said ion-mobility spectrometer.34. The method of claim 13, wherein said pyrolyzer and gas chromatographis connected with a virtual impactor.